The driverless carriages now in use are arranged automatically to follow a path or circuit defined by stationary directors. The directors may, for instance, consist of electric guide cables sunk in the ground or floor, the magnetic field of which actuates a receiver on the carriage, or optical reflectors cooperating with optical path finders equipped with photocells on the carriage, or steel tapes arranged in or on the ground or floor cooperating with electromagnetic sensors on the carriages. A sideways deviation of the carriage from the path defined by the director causes the path finder to actuate the steering means of the carriage in the sense required to bring back the carriage to the predetermined path. The steering of the carriage may, for instance, be effected by means of one or more steering wheels angularly adjustable about a vertical axis, or by means of individual control of the drive of a pair of angularly nonadjustable driving wheels having a common geometrical axis (differential steering).
In existing systems, it sometimes happens that the carriage, on performing automatically a turn or change of direction, gets into a position which deviates so much from the predetermined path that the sensor or receiver of the carriage is out of feeling with the fixed directing means, an accident comparable to the derailment of a rail carriage. Accidents of this kind are particularly liable to occur on slippery spots in which the grip of the wheels on the ground or floor is inadequate to prevent slipping.